This invention relates generally to operational amplifier (op amp) circuits and, more particularly, to op amp circuits having means for reducing the generation of noise.
In the past, op amp circuits have typically included a first amplification stage having a differential pair of matched transistors in a common-emitter configuration. The base elements of the two transistors serve, respectively, as non-inverting and inverting input terminals, and an output signal, proportional to the difference between signals being applied to the two input terminals, is produced at the collector element of one of the transistors. To ensure stability, the gain-bandwidth product of the first amplification stage must ordinarily be limited so that propagation delays in second and subsequent amplification stages won't reduce the phase margin by too much at the unity-gain frequency of the first stage.
The customary technique for limiting the gain-bandwidth product of the first amplification stage is to place a resistor in the emitter circuit of each of the two transistors. Unfortunately, these resistors have an adverse effect on the noise power generated in the first stage, since noise power density is directly proportional to the emitter resistance. Accordingly, designers have been faced with a trade-off between stability, which accompanies use of a relatively high emitter resistance, and low noise, which accompanies use of relatively low emitter resistance. In some prior devices, emitter resistors on the order of 100 ohms have been selected, which has decreased the signal-to-noise ratio by only 0.1 db over what could otherwise be obtained with no resistor at all, and which has limited the gain-bandwidth product of the first stage to an acceptable level, for purposes of stability.
With the advent of relatively low-noise transistors, such as the LM-394 dual NPN transistor produced by National Semiconductor, however, the noise produced by emitter resistors of 100 ohms can be very significant in comparison to the noise produced in the transistors themselves. One technique for properly benefiting from the low-noise capability of these transistors is to reduce the value of the emitter resistor, but, as previously discussed, such a reduction can increase the gain-bandwidth product of the first stage and thereby have an adverse effect on the stability of the op amp.
Thus, there is a need for an op amp circuit that can properly benefit from the use of relatively low-noise transistors, by generating substantially less noise without having any adverse effects on stability. The present invention fulfills this need.